Electrically operated devices that function in a cordless mode typically include a housing which has a chamber for receiving and retaining a removable battery pack. The battery pack completely encloses one or more cells and provides the necessary DC power for operation of the device. Historically, cordless electrically powered devices have included relatively low power devices such as shavers and hand-held calculators. Recently, improvements in battery technology have led to the development of batteries that store more energy and are capable of driving higher power devices. These devices include for example, portable hand-held power tools and appliances operating at power levels from 50 watts up to hundreds of watts. A hand-held power tool is typically powered by a battery pack that comprises a number of batteries connected in series. To provide the higher power levels required by high power devices an increased number of batteries are connected in series resulting in higher input voltages and battery pack volumetric requirements.
Cordless power devices permit work operations to be performed in areas where a conventional AC power source is not available or inconvenient to use. However, the effective charge capacity of the battery pack and the availability of replacement battery packs limit the use of cordless devices. When the battery pack is discharged, it must be recharged or replaced with a fully charged pack.
Both batteries and battery chargers are expensive in comparison to the power device for which they are intended. Batteries for high power applications cost approximately 30% of the cost of the applicable power device. Additional batteries are required to permit cordless mode operation while a battery is recharged and to replace dead batteries. High power levels drawn from batteries during operation of the power tool, the depth of discharge of the battery, the number of charge/discharge cycles, and the speed with which a battery is recharged all contribute to shortening the usable lifetime of a battery. Fast chargers can cost more than the power tool or appliance that is powered by the battery. There are two basic types of battery chargers, trickle chargers and fast chargers. Trickle chargers are significantly less expensive than fast chargers, however a trickle charger requires approximately 1/2 day to recharge a battery pack. A fast charger on the other hand can recharge a battery pack within approximately one hour. Therefore, a trade off must be made between using a trickle charger with a large number of battery packs versus using a costly fast charger with very few replacement battery packs.
It has recently been proposed to provide portable cordless power tools with an optional corded AC converter module that is connected to an AC power source and designed to replace the battery pack. The corded converter module converts power from the AC source to a regulated low-voltage DC level that is usable by the motor of the power device. Such a device allows a tool operator to use the tool in either the cordless battery mode or the corded AC mode as needed. Thus, the availability of such device enables the operator of a cordless tool to complete a project when the battery pack has been discharged, or to continue to use the tool while the battery pack is charging and a fully charged backup battery pack is unavailable. Hence, by using a corded converter module the need for extra battery packs is minimized.
However, the prior art design of a corded converter module is constrained by a number of factors such as the physical envelope, the required output power level, the voltage conversion ratio of the converter, safety requirements to protect the operator from electrical shock, and cost. The envelope of the corded converter module must conform to the envelope of the battery pack with which it is interchangeable. With the increased volumetric requirements for battery packs there is increased volume available for housing a corded converter. The power output level of the converter is directly related to the available volume within the container envelope. The power output levels adequate to drive power devices such as hand held power tools are possible within the physical envelope of commercial battery packs. The voltage conversion ratio of the converter is the ratio between the rectified input voltage and the converter output voltage. The converter output voltage is set to a level roughly equivalent to the battery voltage. The greater the voltage conversion ratio the more difficult it is to accurately regulate the output voltage. The safety regulations are typically met by isolating the operator of the power device from the AC power source. Commercially available systems meet the safety regulations by employing a high frequency power transformer to isolate the output power of the converter module from the relatively high voltage AC input power source. Power transformers are custom devices that are expensive and bulky in comparison with the other electronic devices of the converter module. Attempts to minimize costs of corded converter modules have concentrated on optimizing the output power capability of the converter module for a given power device. By designing the converter module for the minimum output power required to satisfactorily drive the power device, lower cost electronic components can be chosen for the converter.
Operators of cordless power tools already faced with the cost of battery packs and battery chargers must also invest in expensive corded converter modules for their power tools. As an alternative many purchase a corded power tool to use in lieu of the cordless tool when an AC power source is nearby. Attempts to minimize the cost of corded conversion modules have been constrained by the cost of using transformer isolation to meet the government safety requirements. Obtaining further cost reductions by reducing the output power level of a corded converter module would result in under-powered power devices. While the prior art can be used to provide corded converter modules for a handheld power tool, it has not proven capable of providing low cost modules that are convenient to use.